Geology of the Little Sewickley Creek Watershed

The Little Sewickley Creek watershed is located within the Appalachian Plateau geologic province. Bedrock consists of inter-bedded layers of shale, sandstone, limestone and clay approximately 300 million years old. Our deep valleys and hilly topography are the result of stream erosion through the sedimentary rocks.

Although not widely exploited within our LSC watershed, western Pennsylvania is rich in natural resources essential for energy production (coal, oil and natural gas), building and construction (sand and gravel, clay, limestone) and various manufactured products (clay into bricks, sandstone for glass and of course coal and limestone for steel).

The sedimentary rocks in this area are also great for fossil hunters. Areas along creek beds are great places to look for plant, vertebrate, invertebrate and trace fossils. In nearby Robinson Township, the fossilized head of a 300-million-year old carnivorous amphibian was found in a new road cut. Fedexia Striegeli was found by a University of Pittsburgh student and studied and named by a paleontologist at the Carnegie Museum of Natural History.

The shale and clay layers in the bedrock, known as the Pittsburgh Red beds, also cause serious landslide problems in our area. The weak rocks, saturated soils, steep hillsides and freeze-thaw cycles all work together to create many unstable areas that occasionally let loose. We have all been inconvenienced by many closures of Little Sewickley Creek Road due to landslides.

In addition, (there are) many “ancient” slides exist in our area. Some of the natural benches and flat areas along the local streams were actually formed thousands of years ago when huge amounts of sliding rock and soils came to a stop. While they look like a great spot for a house or road, new construction on those areas can reactivate the slides with disastrous results. Since most standard homeowner’s insurance policies do not cover damage from landslides, recognizing these high risk areas is important.

The LSCWA has topographical maps showing slide prone areas within the watershed. They may be found on our website, under “Data Bank”.

Reference: Master’s Thesis by Richard Yeager, University of Pittsburgh: Geology, Landsliding, and Slope Stability in the Little Sewickley Creek Watershed, Allegheny County, Pennsylvania (1981/1982). The report of this study includes topographical maps showing contours, slide-prone areas, tributaries, springs, and other geological/geographical features.

Last updated: March 27, 2022 at 10:01 am